A redox flow cell (also called a redox flow battery) is an electrochemical system which allows energy to be stored in two electrolytes containing different redox couples with electrochemical potential sufficiently separated from each other to provide an electromotive force to drive the oxidation-reduction reactions needed to charge and discharge the cell.
A redox flow cell comprises a positive compartment and a negative compartment. The positive compartment contains an electrolyte containing redox ions which are in a oxidised state and are to be reduced during the discharging process of the redox flow cell, or are in a reduced state and are to be oxidised during the charging process of the redox flow cell, or a mixture of such ions. The electrolyte in the positive compartment is in electrical contact with a positive electrode. The combination of the positive compartment, the electrolyte and the positive electrode is referred to as the “positive half-cell”. The negative compartment contains an electrolyte containing redox ions which are in a reduced state and are to be oxidised during the discharging process of the redox flow cell, or are in an oxidised state and are to be reduced during the charging process of the redox flow cell, or a mixture of such ions. The electrolyte in the negative compartment is in electrical contact with a negative electrode. The combination of the negative compartment, the electrolyte and the negative electrode is referred to as the “negative half-cell”. The electrolyte in the positive compartment and the electrolyte in the negative compartment are separated by an ionically conducting separator, typically an ion exchange membrane, to provide ionic communication between the electrolyte in the positive compartment and the electrolyte in the negative compartment.
Of the redox flow cells developed to date, the all-vanadium redox flow cell has shown long cycle life and high energy efficiencies of over 80% in large installations of up to 500 kW in size. The all-vanadium redox flow cell contains the V(V)/V(IV) redox couple in the positive half-cell electrolyte, and the V(III)/V(II) redox couple in the negative half-cell electrolyte. While the performance characteristics of the all-vanadium redox flow cell have made it well suited to various stationary applications, its relatively low energy density has to date limited its application in some fields, for example, in electric vehicles or other mobile applications.
The energy density of a redox flow cell is related to the concentration of the redox ions in the electrolyte in both half-cells, the cell potential and the number of electrons transferred during discharge per mole of active redox ions. V(V) salts have low solubility in most electrolytes. For all-vanadium redox flow cells, the highest concentrations of V(V) ions achieved to date have been achieved using sulphuric acid as the supporting electrolyte. In the case of the all-vanadium redox flow cell, the maximum vanadium ion concentration that can be employed for wide temperature range operations is typically 2 M or less. This concentration represents the solubility limit of the V(II) and/or V(III) ions in the sulphuric acid supporting electrolyte at temperatures below 5° C. and the stability of the V(V) ions in the sulphuric acid supporting electrolyte at temperatures above 40° C. V(V) ions in a sulphuric acid solution are subject to thermal precipitation at temperatures over 40° C.
Aqueous hydrochloric acid is unsuitable for use as the supporting electrolyte in all-vanadium redox flow cells as V(V) ions are reduced by chloride ions giving rise to chlorine gas and the formation of V(IV) ions.
It would be advantageous to develop alternative redox flow cells, which, in at least preferred embodiments, can achieve a higher energy density than conventional all-vanadium redox flow cells.